The majority of colorectal cancers have mutations of the ARC (adenomatous polyposis coli) tumor supressor gene or the ?-catenin oncogene, causing ?-catenin accumulation that activates other oncogenes. This project will investigate the interaction of ?-catenin and other signaling molecules in the intestine. Gut-enriched Kr[unreadable]ppel-like transcriptional factor (GKLF, now called KLF4) is down-regulated in many colon cancer cells and considered to be a candidate tumor suppressor protein;it inhibits intestinal cell proliferation and induces cell differentiation. In contrast, ?-catenin induces intestinal cell proliferation and inhibits differentiation. KLF4 expression is APC-dependent, and its function and expression are inversely correlated with those of ?-catenin. The central hypothesis of this project is that KLF4 inhibits ?-catenin signaling in the differentiated cells;conversely, ?-catenin inhibits KLF4 expression in proliferating cells, so KLF4/??-catenin crosstalk defines the proliferating and differentiating compartments in the intestine. Up- regulation of ?-catenin and down-regulation of KLF4 together contribute to the initiation of colon cancers. To address this hypothesis, we will pursue three Specific Aims, to: 1) Delineate the mechanisms by which KLF4 inhibits ?-catenin signaling using cell-based assays or in vitro-binding assays;2) Determine the mechanisms of APC-regulated KLF4 expression in colon cancer cells and in the intestines of APC-deleted mice;and 3) assess the functional effects of KLF4/??-catenin interactions in human colorectal cancers and in nude mice injected with colon cancer cells with altered KLF4/??-catenin signaling. These experiments should elucidate the role of this important signaling axis in the development and progression of colorectal cancers, and eventually lead to new clinical markers and treatment approaches for this important class of cancers. Health Relevance: Colorectal cancer is the second leading cause of cancer-related deaths in the U.S., with -150,000 new cases and 57,000 deaths annually (about 10% of all cancer deaths), at an estimated annual economic cost greater than $3.5 billion. A better understanding of the signaling pathways contributing to colorectal cancer progression should provide targets for novel therapeutic agents to treat this disease.